Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a transmission line probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank.
An electromagnetic transmit signal is generated by a transceiver and propagated towards the surface of the product in the tank, and an electromagnetic reflection signal resulting from reflection of the transmit signal at the surface is propagated back towards to the transceiver.
Based on a relation between the transmit signal and the reflection signal, the distance to the surface of the product can be determined.
The transmit signal is typically not only reflected at the impedance transition constituted by the interface between the tank atmosphere and the surface of the product, but at several other impedance transitions encountered by the transmit signal. These impedance transitions may, for example, result from fixed structures in the tank, such as the feed-through through the tank roof.
Impedance transitions resulting from fixed structures in the tank may make it difficult to accurately determine the filling level when the filling level is close to the fixed structure.
To improve this situation, it has, for example, been proposed to subtract a representation of a reference measurement (for instance taken when the tank is empty) from the representation of a present measurement.
This approach, which is for example described in U.S. Pat. No. 6,078,280 may generally improve the filling level measurement result, but may be sensitive to changes in the environment, such as temperature etc, occurring between the time when the reference measurement was performed and the time of the present measurement.
It would therefore be desirable to provide for improved filling level measurement, in particular a more robust filling level determination.